Re: 20 Inch Occ Schwinn Stringray 5 Speed?
Brettmavrik set his Occ up with a 5-speed cluster and a deraileur. It was easy. The sprockets thread on. Use a punch and hammer to knock the original, single sprocket, freewheel loose from the hub. Thread the 5-speed cluster on. You will need to spread the rear of the frame about 3/8" so that you can put the wheel back on.
If I remember correctly the deraileur bolts right on without any modifications.
I you are after a 5-speed pedal bike, I don't think the whole change-over will take more than a few hours. Freewheel swap, add a deraileur, alter the chain length, and mount a shifter.
If you want a shifter set-up for your engine, it get's more complicated.